(1) Field of the Invention
The invention relates to a audio output device circuit, and more particularly, to superpositioning of different audio input channels as voice, music and ringer signals wherein said different audio signals are using a common output stage and can be heard together or separately as required.
(2) Description of the Prior Art
In prior art a summation point as input to an amplifier performs the “mixing” of different audio signals as voice, music or ringer signals. FIG. 1 shows a typical prior art implementation of the superpositioning of two audio signals as input, e.g. from a voice signal 1 and a ringer signal 2. Both input signals can be activated either separately or together by two switches 3 and 4. Point 5 is the stable point in this configuration with the property of a virtual ground. The input of the amplifier 6 comprises the summation point 5 and a reference voltage 8. Typically said reference voltage 8 is identical at each stage. The resistor 7 is controlling the gain of the amplifier 6. The resistor 9 provides the entry resistance of the following output stage comprising the amplifier II, the gain adjustment resistors 10, said reference voltage 8 and the output voltage 12 which can be used to drive a loudspeaker. It is obvious that with this configuration the offset caused by amplifier 6 will be multiplied by the gain of the output stage.
The patent application (DS01-024) by the same inventors Klaus Graef and Edgar Sexauer Ser. No. 09/995,195 filed Nov. 27,2001 describes a circuit wherein the same output stage is used for either the audio mode or the ringer mode of a mobile phone. The gain adjustment is done using amplitude setting by the same string of resistors in the audio mode as well as the ringer mode. “Mixing” or superpositioning of different audio sources is not covered and shown in cited patent application.
U.S. Pat. No. 5,541,543 to Arnaud discloses a telephone set fitted with a speech signal loudspeaker also serving as a ringer and more particularly to a device for regulating the power provided to the loudspeaker during a ring signal.
U.S. Pat. 5,894,212 to Wendt et al. shows a ring signal from a DC power supply using a pair of switches controlled by a pulse width modulation (PWM) circuit.